The 1980s were a very exciting time for mesoscopic physics characterized by a fruitful interplay between theory and experiment. What emerged in the process is a conceptual framework for describing current flow on length scales shorter than a mean free path. This conceptual framework is what we have tried to convey in this book. The activity in this field has expanded so much over the last few years that we have inevitably missed many interesting topics, such as persistent currents in normal metal rings, quantum chaos in microstructures, etc.

The development of the field is far from complete. So far both the theoretical and the experimental work has been almost entirely in the area of steady-state transport and many basic concepts remain to be clarified in the area of time-varying current flow as well as current fluctuations. Another emerging direction seems to be the study of mesoscopic conductors involving superconducting components. Finally, as we study current flow in smaller and smaller structures it seems clear that electron–electron interactions will play an increasingly significant role. As a result it will be necessary to go beyond the one-particle picture that is generally used in mesoscopic physics. Single-electron tunneling is a good example of this and it is likely that there will be many more developments involving current flow in strongly correlated systems.